Quality printing method, printing machine, and corresponding printing substance

ABSTRACT

A printing process for the transfer of printing substance ( 2 ) from an ink carrier ( 1 ) to an imprinting material ( 6 ), in which, with the help of an energy-emitting apparatus, which, during a process period, emits energy in the form of electromagnetic waves ( 3 ), and the printing substance ( 2 ) undergoes a change in volume and/or position, wherein, with the help of absorption bodies ( 4 ), energy is transferred from the electromagnetic waves ( 3 ) into the printing substance ( 2 ). The invention also includes an apparatus for practicing the process of the invention and a printing substance containing absorption bodies.

BACKGROUND OF THE INVENTION

The present invention relates to a printing process for the transfer ofprinting substance from an ink carrier onto an imprinting material, inwhich, with the help of an energy-emitting apparatus which emits energyduring a process period in the form of electromagnetic waves, theprinting substance undergoes a change in volume and/or position, and forexample as a result a transfer of a printing point onto the imprintingmaterial takes place, as well as a printing machine and a printingsubstance for this.

By a printing process is meant primarily a process for the reproductionas often as required of text and/or image patterns by means of aprinting plate which is re-inked after each impression. In general, adistinction is made here between four basically different printingprocesses. Thus firstly the relief printing process is known, in whichthe printing elements of the printing plate are raised, while thenon-printing parts are recessed. This includes for example letterpressprinting and so-called flexographic or aniline printing. Furthermore,flatbed-printing processes are known in which the printing elements andthe non-printing parts of the printing plate essentially lie in onelevel. These include offset printing in which strictly speaking theinked drawing on the printing plate is not printed directly onto theimprinting material, but is first transferred onto a rubber cylinder ora rubber blanket and only then is the imprinting material printed fromthis. Where in the following reference is made to imprinting material,however, this is to be understood as both the actual imprintingmaterial, i.e. the material to be printed on, and any chosen transfermeans, such as e.g. a rubber cylinder. A third process is the so-calledgravure printing process in which the printing elements of the printingplate are recessed. A gravure printing process used industrially isso-called rotogravure printing. Finally, a porous printing process isalso known in which at the printing positions the ink is transferredonto the imprinting material through screen-like openings of theprinting plate.

These printing processes are all characterized by the fact that theyrequire a printing plate which was more or less costly to produce, withthe result that these printing processes operate profitably only withvery long print runs, usually well over 1000 units.

For the printing of short print runs, printers are already used whichare often connected to an electronic data processing system. These usegenerally digitally triggerable printing systems which are in a positionto print individual printing points as required. Such printing systemsuse various processes with different printing substances on differentimprinting materials. Some examples of digitally triggerable printingsystems are: laser printers, thermal printers and ink jet printers.Digital printing processes are characterized by the fact that they donot require printing plates.

Thus for example an electro-thermal ink jet printing process is knownfor example from GB 2 007 162, in which the water-based ink is brieflyheated in a suitable ink jet by electrical pulses until it boils, withthe result that a gas bubble suddenly forms and an ink drop is ejectedfrom the jet. This process is generally known by the term “bubble-jet”.However, these thermal ink printing processes have in turn thedisadvantage that on the one hand they consume a great deal of energyfor the printing of an individual printing point and on the other handthey are suitable only for printing processes which are water-based.Furthermore, every single printing point must be triggered separately bythe jet. On the other hand, piezoelectric ink printing processes sufferfrom the disadvantage that the required jets are easily blocked, withthe result that only special and expensive inks can be used for this.

It is known from DE 197 46 174 that a laser beam, through very shortpulses in a printing substance which is located in cells of a printingroller, induces a procedure with the result that the printing substanceundergoes a change in volume and/or position. As a result, the printingsubstance spreads over the surface of the printing plate and it ispossible to transfer a printing point onto an imprinting material movedup against same. However, in this process it is disadvantageous that thefilling of the cells is very difficult due to the small diameter of thecells. Therefore it is proposed in DE 100 51 850 to apply the printingsubstance essentially forming a continuous film to the ink carrier. Theenergy can be either transferred directly into the printing substance orfirstly into an absorption layer which is applied to the ink carrier,and which in turn emits the energy to the printing substance. In thefirst case, special printing substances must be used which are capableof absorbing the energy. This severely restricts the variety of printinginks that can be used. In addition, the absorption of the light in theprinting ink takes place within a relatively large volume passed throughby the laser beam. With some inks the energy is also not completelyabsorbed. The absorption is also strongly dependent on the printingsubstance used and the actual thickness of the printing substance on theink carrier. Due to the relatively large volume in which the energy isabsorbed, a relatively large amount of energy must be introduced intothe printing substance in order to induce the change in volume and/orposition of the printing substance necessary to set a printing point.Moreover, a delay in boiling often occurs with the result that thetemperature at which gas bubbles form in the printing substance cannotbe predicted. This means that the absorption—and the local heating ofthe printing substance associated therewith—takes place largelyuncontrolled, which results i.a. in a marked variation in printing-pointsize. To ensure that the desired printing point is set in each case,much more energy must therefore be introduced into the printingsubstance than is usually necessary to induce the desired change inposition and/or volume of the printing substance.

If on the other hand the printing substance is heated indirectly byintroducing the energy firstly into an absorption layer, the requiredenergy is generally even higher as, in addition to the printingsubstance, the absorption layer must also now be heated. In addition,the heat conduction in the absorption layer must not be ignored, withthe result that a larger area of the absorption layer is necessarilyheated, which likewise increases the required energy. Moreover, with theknown processes, even a detachment of the absorption layer often occurs.

BRIEF DESCRIPTION OF THE INVENTION

The object of the present invention is therefore to provide a printingprocess which has an improved printing quality and which requires muchless energy for printing a printing point, but which at the same timeallows a high printing speed with high resolution capacity. The objectof the present invention is also to provide a corresponding printingmachine and a corresponding printing substance with which the printingprocess according to the invention can be realized.

More particularly, the invention includes a printing process for thetransfer of printing substance (2) from an ink carrier (1) to animprinting material (6), in which, with the help of an energy-emittingapparatus, which, during a process period, emits energy in the form ofelectromagnetic waves (3), and the printing substance (2) undergoes achange in volume and/or position, wherein, with the help of absorptionbodies (4), energy is transferred from the electromagnetic waves (3)into the printing substance (2). The invention also includes anapparatus for practicing the process of the invention and a printingsubstance containing absorption bodies.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A through 1C show schematic representations of the operatingprinciple of a first version of the process,

FIGS. 2A and 2B show a schematic representations of the operation of aprocess of the state of the art,

FIGS. 3A and 3B show a schematic representations of an advantageouseffect of the first version of the printing process according to theinvention,

FIG. 4A through 4C show a schematic illustrations of a second version ofthe process according to the invention,

FIGS. 5A and 5B show a schematic representations of the influence of thecorrect energy parameters,

FIGS. 6A and 6B show a schematic representations of the influencing ofthe printing-point size,

FIG. 7 shows a schematic representation of the influence of themicrostructure on the printing point,

FIG. 8 shows a schematic representation of the influence oflight-focussing elements,

FIGS. 9A and 9B show exemplary arrangements of the light-focusingelements,

FIG. 10 shows another version in a “negative” arrangement and

FIG. 11 shows another version with an additional, interposed inkcarrier.

DETAILED DESCRIPTION OF THE INVENTION

As regards the process, the object is achieved in that the energy istransferred from the electromagnetic radiation into the printingsubstance with the help of absorption bodies.

Without absorption bodies, the electromagnetic wave penetrates far intothe printing substance and is absorbed along a relatively large area. Tonow achieve the necessary change in volume and/or position of theprinting substance, a relatively large area of the printing substancemust be heated until a gas bubble forms. The effect of using absorptionbodies is that the electromagnetic wave is absorbed essentially at thesurface of the absorption body, i.e. over a much smaller area. As aresult, even with a very much smaller energy contribution, a heatingoccurs at certain points with the result that the desired change involume and/or position of the printing substance already takes placeearlier. Moreover, a delay in boiling can be effectively preventedthrough the presence alone of the boundary surfaces between printingsubstance and absorption bodies which act as a type of crystal nucleus.

Through the use of absorption bodies, the required energy can thus bedrastically reduced. There is also a controlled change in positionand/or volume of the printing substance, which moreover scarcely stilldepends on the printing substance used.

Another preferred version of the process according to the inventionprovides that through the induced change in position, some of theprinting substance is detached from the ink carrier and the ink carrierthan comes directly into contact with the imprinting material with theresult that, at those points at which a change in position has beeninduced, no transfer of printing substance onto the imprinting materialtakes place. In other words, with this version, the electromagnetic waveis directed to those areas which are not to transfer printing substance.

In a particularly preferred version of the present invention, absorptionbodies are used which are smaller than the wavelength of theelectromagnetic waves, preferably smaller than a tenth, particularlypreferably smaller than a fiftieth of the wavelength of theelectromagnetic waves.

Through the use of such small absorption bodies, the absorption effectis further increased as the waves are no longer scattered at suchsmaller bodies, but exclusively absorbed.

Advantageously, the energy-emitting apparatus emits energy in the formof laser light. With the help of highly coherent monochromatic laserlight, a relatively high amount of energy can be emitted onto a verysmall area with very short light pulses. As a result, the quality of theprint format and in particular the resolution is increased. A shortlight pulse need not necessarily come from a pulsed laser. It is,rather, actually advantageous if a laser is used in CW operationinstead. The pulse duration or better the exposure time does not thendepend on the length of the laser pulse but on the scanning speed of thefocus. Moreover, the data to be transferred need no longer besynchronized to the fixed pulse frequency.

In a further particularly preferred version it is provided that aprinting substance is used which contains the absorption bodies. As aresult, the area in which the electromagnetic waves are absorbed can bereduced still further, which leads to a further reduction of therequired energy supply.

A further particularly preferred version provides that the absorptionbodies absorb essentially all the light wavelengths. This ensures thatthe absorption is as complete as possible.

However, the consequence of this, in particular when the absorptionbodies are contained in the printing substance, can be that not allcolours, in particular no bright colours, can be used as the colour ofthe printing substance is changed by the addition of the absorptionbodies which absorb essentially all the light wavelengths.

Therefore it is provided in another particularly preferred version thatthe absorption bodies essentially absorb only radiation with awavelength or in a wavelength range which corresponds to the wavelengthor the wavelength range of the electromagnetic waves emitted by theenergy-emitting apparatus. The absorption bodies thus merely filter thelight of the energy-emitting apparatus out of the incident light, whileall the other light wavelengths can pass unhindered, with the resultthat the influence of the absorption bodies on the colour of theprinting substance is negligible. Through this measure, the variety ofthe printing substances to be used can be clearly increased, as theabsorption bodies need merely be added to the printing substances.

Naturally it is also possible, through a suitable choice of thewavelength range in which incident light is absorbed, to also use theabsorption bodies as a dye or pigment.

In a particularly preferred version of the present invention, theenergy-emitting apparatus or the beam path of the electromagnetic wavesare arranged such that the absorption bodies are accelerated through theelectromagnetic waves of the energy-emitting apparatus in the directionof the imprinting material. As a result the change in volume and/orposition of the printing substance is supported in advantageous manner.The acceleration of the absorption bodies alone results in a type ofshock wave in the printing substance with the result that, incombination with the forming gas bubble, a defined drop detachment isfavoured. This can be realized for example in that the ink carrier isdesigned transparent and the light beam from the side of the ink carrierfacing away from the printing substance is focussed through the formerinto the printing substance. On the side of the absorption body facingthe ink carrier, a gas bubble then forms explosively which ensures theacceleration of the absorption body in the direction of the imprintingmaterial.

Naturally it is not absolutely necessary for the absorption bodies to bearranged in the printing substance. In particular when using transparentprinting substances an ink carrier is used according to the invention onthe surface of which, provided to receive the printing substanceabsorption bodies are present which preferably form a solid layer.

The only essential is that the printing substance comes into contactwith the absorption body as directly as possible. However the printingsubstance need not surround the absorption bodies completely. Becausethe energy is now no longer absorbed in an absorption layer, but on anabsorption body, the surrounding area is heated only very slightly. Itis thus possible through the process according to the invention to heatthe—preferably very small—absorption body in a targeted manner withoutlosses occurring due to heat conduction.

Because a targeted and reproducible change in position and/or volume ofthe printing substance can be induced with the help of the absorptionbodies, the printing-point size can now advantageously be controlledthrough the amount of energy emitted by the energy-emitting apparatus.This control can be effected for example by varying the process periodor the pulse duration. Although such a control has already been proposedin principle in DE 100 51 850, it was not practicable without the use ofabsorption bodies as the uncontrolled scatter of the printing-point sizeoverlays the control of the printing-point size via the pulse duration.

When using laser light in another particularly preferred version theprinting-point size can also be controlled by the profile of the laserlight. By profile of the laser light is meant various transverseelectromagnetic modes TEM.

Differences in brightness of the image to be printed can advantageouslybe achieved by variation of the printing-point size.

Surprisingly, tests have shown that the best printing results areachieved when the process period or the pulse duration is shorter than 1μs, preferably shorter than 250 nanoseconds, particularly preferablyshorter than 100 nanoseconds and best of all shorter than 50nanoseconds.

Moreover it is particularly preferred if during the process period orduring the pulse duration an energy density higher than 500 kW/cm²,preferably higher than 2 MW/cm² and particularly preferably higher than10 MW/cm² is achieved at the surface of the irradiated absorption body.

Furthermore, tests have shown that the best printing quality can beachieved when the radiation-exposed surface of the absorption body isheated during the process period or pulse duration at an average heatingrate greater than 10⁹ K/s, preferably greater than 10¹⁰ K/s,particularly preferably greater than 10¹¹ K/s.

Through the last-named measures a very rapid, point-precise heating ofthe absorption body results, with the result that—in contrast to theprocesses of the state of the art—a shock wave and not a sound wave isproduced, which leads to a clear improvement of the printing quality.

The above measures for improving the printing quality are also based,among other things, on a novel raster method, which is preferably usedin the present invention, but which in principle can also be usedindependently of the other features of the present invention. Thisconsists of a line spacing in which printing occurs line-by-line and theindividual lines have a very tight distance between each other and eventouch in an ideal case, with the result that in vertical direction theresolution is set by the distance between lines or the line width.However no fixed point raster is provided in horizontal direction.Instead the “pixels” consist in horizontal direction of line segments ofthe respective raster lines of varying length with any chosen startingand end point. The scanning of the individual lines preferably takesplace with a continuous wave laser which according to requirements issimply masked or switched off or its energy is reduced, so as to notprint individual sections of the raster lines, while those line segmentswhere the laser is not masked or turned off are printed. However, thelaser itself continues to operate in continuous wave mode in principle.The length of a printed line segment depends simply on the length of theswitching-on or revealing time of the laser on the respective line. Aprinted line thus consists of a sequence of line segments which ingeneral are of different length, their length being determined simply bythe switching-on time of the laser, the individual line segments havinga line width which essentially remains constant, which ideallycorresponds exactly to the distance between adjacent scanning lines orcan even be slightly larger (or smaller).

However these lines have at their ends small “run-outs” or “roundings”,which correspond to the rise time or fall time of the laser duringswitching on or off or during masking or revealing. Only if theswitch-on period or the exposure period for a specific line segmentbecomes shorter than the rise time and/or fall time at the ends of therespective line segments is the width of the printed line also reducedwith the correspondingly reduced length of the printed “line”, with theresult that when switch-on time is extremely short only points areprinted and when switch-on times vary on in the range of the rise andfall times, points of different sizes or different diameters below theline width can be printed. In principle any chosen and very fine shadesof grey can be produced in this way.

The position of the individual line segments, i.e. in particular theirstart and end, is not specified by any one grid, but can be varied asdesired. In this respect the corresponding printing process is strictlyspeaking screened only in vertical direction, i.e. perpendicular to thepattern of the individual printed lines, while a “continuous” printingtakes place within the lines, i.e. in principle a continuous line isprinted which can however be interrupted at any chosen points and forany lengths, with the result that the printed line sections arecorresponding line segments of constant width and any choosable length.Only when the length of these line segments is comparable with theirwidth or even shorter is the width of these lines also reduced, with theresult that the corresponding line segments are then shortened toindividual points, which can also be reduced even further in theirdiameter measured in width direction of the lines by further reducingthe switch-on time of the laser.

The laser used is preferably a phase-coupled or a CW laser with anaverage output of more than 10 watts and a beam parameter M²<1.5. The“switching-on” and “switching-off” of the laser expediently takes placevia a pulse-width modulator in combination with a suitable laser switch(e.g. AOM, EOM). However, in the preferred variant of the invention thelaser beam is not fully “turned off” but is only reduced in its energyor energy density to below a threshold limit beneath which there is nodetachment of dye drops from the ink carrier. For example there is areduction in the laser output to 15% of the value used for a fullprinting point. This simplifies the control and monitoring of the laserenergy when printing, in particular an improved and more effective useof the laser switches or modulators is thereby made possible. In thecase where an AOM “switch” is used the laser can thus be used at order0, while conventional applications of AOM switches have to use the1^(st) diffraction order. Advantageously the laser has a wavelengthbetween 0.5 μm and 3 μm.

In addition the selected thickness of the printing substance on the inkcarrier should be less than 50 μm, preferably less than 30 μm,particularly preferably less than 20 μm. The thickness of the printingsubstance layer should not fall below 5 μm, however. The optimal rangeis between 10 and 15 μm.

Advantageously the printing substance is selected such that theviscosity lies between 0.05 and 0.5 Pas.

The energy required can be even further reduced if absorption bodies andprinting substance are selected such that the absorption bodies arewetted as well as possible by the printing substance. It is therebyensured that the energy applied to the absorption body is dissipated inthe printing substance as directly as possible.

As regards the printing machine, the object mentioned at the start isachieved by means of a printing machine for printing on an imprintingmaterial which has an ink carrier and an energy-emitting apparatus whichis arranged such that the energy can be transferred in a targeted mannerto certain areas of the ink carrier, absorption bodies being providedfor absorbing the energy. The energy-emitting apparatus is preferably alaser.

Preferably the absorption bodies have a size smaller than 1 μm,preferably smaller than 200 nanometers and particularly preferably asize between 10 and 50 nanometers. With such small absorption bodies itis ensured that almost no light scattering occurs. The absorption bodiespreferably consist of carbon black particles.

The absorption bodies are preferably arranged in an absorption layerapplied to the ink carrier.

In a particularly preferred version the proportion of absorption bodiesin the absorption layer is more than 40 wt.-%, preferably more than 70wt.-% and particularly preferably more than 90 wt.-%.

In another particularly preferred version the absorption layer consistsof pressed absorption bodies.

The absorption bodies can also be embedded in an organic or inorganicpolymer matrix, such as e.g. polysilicates but also epoxy resin. It isalso possible to embed the absorption bodies in an elastomer matrix, forexample rubber. It is thereby ensured that the absorption bodies arearranged largely separated from one another.

An expedient version provides that the ink carrier consists of atransparent polymer strip (polyester), because there is then a goodwettability for printing substances based on an alcoholic, aqueous oraromatic solvent.

As regards the printing substance, the object mentioned at the start isachieved by a printing substance which contains absorption bodies.

These absorption bodies advantageously are of a size smaller than 1 μm,preferably smaller than 200 nm and particularly preferably between 10and 50 nm.

In particularly preferred versions of the printing substance theprinting substance consists of binder, dye or pigment, solvent,additives and the absorption bodies.

In another particularly preferred version the absorption body forms thedye, i.e. the absorption body has a natural frequency band in thevisible region, with the result that selectively determined frequencyareas are absorbed.

A further advantageous version provides that the printing substance hasa blowing agent. A blowing agent is an agent which boils at a relativelylow temperature, so that gas bubbles already form at a low temperature.A further reduction of the energy to be supplied can thereby beachieved. Printing substances which boil at a temperature≦100° C. areparticularly suitable. Advantageously the blowing agent is used assolvent at the same time. Alcohols, esters, ketones and water amongothers may be used as blowing agent. In the case of solvent-free inks,e.g. offset inks or UV inks, blowing agents are expediently added.

Expediently more than 10 wt.-%, preferably more than 12 wt.-% andparticularly preferably more than 15 wt.-% blowing agent is added. Theblowing agent content should be less than 30 wt.-% overall, however.

According to the invention the described printing substance is used inthe described printing process according to the invention and/or in thedescribed printing machine according to the invention.

In the preferred version of the invention the printing machine containsa printing carrier which has a surface structure consisting of recessesand elevations, which is designed such that it acts at least partlyfocussing on a shock wave produced on or immediately above the surfaceof the ink carrier. These microstructures, e.g. recesses or elevations,are firstly to be smaller than a typical laser beam diameter which isused to produce the shock wave by local heating of the printingsubstance and they should above all be arranged periodically in at leastone dimension, with the period also having a shorter repetition intervalthan corresponds to the typical diameter of the laser beam. It is alsosufficient if there is also microstructurization in essentially only onedirection, i.e. if the microstructures consist of e.g. grooves or ribs,which typically run perpendicular to the scanning direction of the laserbeam. Preferably the recesses or elevations have a diameter or distanceof the order of magnitude of 30μ and smaller, preferably smaller than 15μ. The focussing effect is achieved for the pressure waves in a similarway as for the electromagnetic, in particular optical waves.Specifically, therefore, slightly concave shapes, e.g. grooves with anelliptical or concave, optionally also circular cross-section, areprovided, where the focussing direction should run as perpendicular aspossible to the enveloping surface of the ink carrier, in order that thedye drops ejected essentially perpendicular to the enveloping surface ofthe ink carrier by a correspondingly directed shock wave have as short apath as possible to the imprinting material and thus have as small aspossible a divergence.

The exact design of the recesses depends among other things on theirradiation direction of the laser beam and on whether the laser beam isdirected onto an absorption layer or whether absorption bodies arecontained in the printing ink itself.

It is to be borne in mind, as already mentioned, that in contrast to thestate of the art, which partly also already shows surface structures,the surface structures according to the invention have considerablysmaller characteristic dimensions, with the result that a specificprinting point thus generally comprises several adjacent structuralelements.

The focusing effect of specific structures can also theoretically bedetermined according to the rules of optics or optical reflection and/orrefraction. The result of a laser beam striking in the area ofcorresponding structural elements is a very sudden local heating and/orevaporation of the blowing agent, with the result that an explosivepressure wave develops at the site of the heating. This pressure wave iseither produced at the base of the ink carrier itself or is reflectedthere, with the result that on this basis the preferable alignment andfocussing of the pressure wave in a preset surface structure can alreadybe easily estimated. Although smaller scatter proportions can never becompletely avoided, these are generally not important, especially when,because of the low energy of this scatter proportion, this energy is notsufficient to detach small ink drops or if the surface tension of theink drop is sufficient to essentially hold the latter together and todetach it only in the main direction of the shock wave.

The microstructurization and the restriction of the microstructures tocharacteristic dimensions, which are even smaller than the diameter ofthe laser beam scanned away via the ink carrier are thus also importantbecause, in contrast to the state of the art, the afterflow of ink intothe microstructures is to be as unrestricted and rapid as possible.Because of the small size of the microstructures the ink layer is alsoalways thicker than the depth of the microstructures, with the resultthat for these reasons alone the individual recesses cohere over the inklayer.

It is understood that elevations attached in a targeted mannerperiodically and on a small scale can also be designed such that theyhave a corresponding focussing effect.

Additional advantageous designs are given in the dependent claims.

Other advantages, features and possible applications of the presentinvention will become apparent from the following description ofpreferred versions and from the associated figures.

FIG. 1A shows diagrammatically the situation in the case of a printingprocess of the state of the art. A laser beam 3 is focussed into theprinting substance 2 through the ink carrier 1. The laser beam thuspasses through the ink carrier into the printing substance and isabsorbed over a large area. The result is that the energy requirementfor printing a point is very high, which slows down the printing processand thus reduces the printing speed.

FIG. 1B shows diagrammatically a first version of the printing processaccording to the invention. Here, absorption bodies 4 are introducedinto the printing substance 2. Here, the laser beam 3 now ensures thatthe surface of the absorption body 4 heats up strongly, with the resultthat the energy introduced by the laser beam 3 essentially leads to apunctiform heating of the absorption body 4. Absorption body 4 andprinting substance 2 are advantageously selected such that the printingsubstance 2 wets the absorption bodies 4 very satisfactorily, with theresult that the absorption bodies 4 and the printing substance 2 producea homogeneous suspension. Through the essentially punctiform heating ofthe absorption bodies 4, an essentially punctiform heating of theprinting substance 2 is achieved at the point at which the printingsubstance 2 wets the absorption body 4. In the version shown in FIG. 1,the absorption bodies have an average size of approximately 1 μm.

FIG. 1C shows essentially the same situation as FIG. 1B, except thathere the absorption bodies are very much smaller, namely very muchsmaller than laser light wavelength. Through the very much smallerabsorption bodies 4, the absorption effect is further enhanced as, whenthe light beam 3 strikes an absorption body 4, only light absorption isstill brought to bear, but not light scatter.

The satisfactory wetting of the absorption bodies 4 by the printingsubstance can be achieved for example in that both absorption bodies 4and also the printing substance 2 are designed polar, e.g. an alcoholcan be used as solvent. Because of the electrostatic attraction, thisautomatically leads to satisfactory wetting. On the other hand, if theprinting substance is designed less polar, if for example benzene ortoluene is used as solvent, then the absorption bodies should also be asnon-polar as possible, in order to guarantee satisfactory wetting.

In particular in the case of dark printing substances 2 the absorptionbody 4 can be developed such that it absorbs light with essentially allthe light wavelengths. The result is the most complete possibleabsorption of the emitted laser light by the absorption bodies. Howeverit is also possible to develop the absorption body 4 such that itabsorbs only light of the corresponding laser wavelength. As analternative to this, the absorption bodies 4 can also absorb light ofwhole wavelength ranges, with the result that the absorption bodies 4are used simultaneously as dye or pigment.

A further advantage which is achieved by the use of the absorptionbodies 4, is illustrated with reference to FIGS. 2A and 2B in comparisonwith FIGS. 2A and 2B. FIGS. 2A and 2B once again show diagrammatically aprinting process known from the state of the art. In FIG. 2A, it can beseen that the printing substance 2 is applied to the ink carrier 1. If,as shown in FIG. 2A, a laser beam is focussed into the printingsubstance through the ink carrier 1, vapour bubbles 8 form in theprinting substance 2. As no absorption bodies 4 are present, the energyof the laser beam 3 is absorbed over a large area, with the result thatthe energy requirement is very high. The forming vapour bubbles 8 inaddition emit a printing pulse essentially in all directions, which canlead to uncontrolled disturbances in the movement of the printingsubstance 2, with the result that in general no defined drop can becomedetached, but the printing substance is sprayed in uncontrolled manner.

If therefore, as shown in FIG. 3A, there are absorption bodies 4 in theprinting substance 2, less thermal energy is required for the ejectionof a drop from the printing substance 2. This is because, firstly, thethermal energy is essentially absorbed on the surface of the absorptionbodies 4, i.e. within a very small volume and, secondly, the surfaces ofthe absorption bodies 4 form a kind of “crystal nucleus”, which ensurescontrolled formation of gas bubbles. As shown in FIG. 3B, duringirradiation of the absorption bodies 4 by a laser beam 3 on the side ofthe absorption bodies 4 which the laser beam 3 strikes, a defined bubbleformation occurs, which in turn accelerates the absorption bodyessentially in one direction, namely in the direction of the irradiatedlaser beam. Through this movement of the absorption bodies 4 the definedchange in volume and/or position of the printing substance 2 is furthersupported, with the result that a defined drop detachment is favoured.The printing process as a whole is clearly shortened and can be carriedout with much less energy.

FIGS. 4A to 4C show a second version of the process according to theinvention. FIG. 4 a once again shows diagrammatically a printing processfrom the state of the art. Here a laser beam 3 is passed through theprinting substance 2 onto an absorption layer 4, which is applied to anink carrier (not shown). The absorption layer 4 is clearly heated by thelaser beam 3. However this does not take place just in punctiformmanner, as the heat can spread relatively rapidly in the absorptionlayer.

According to the invention it is therefore proposed that, as shown inFIGS. 4B and 4C, the absorption layer 4 contains absorption bodies 8.Here, too, the effect of the absorption bodies 8 is that the heat isessentially focussed in a limited manner on a local area, with theresult that in a very short time the detachment of a drop from theprinting substance 2 can be achieved with relatively little energy, assymbolized by the inkjet 5.

With this version also, it is advantageous to ensure that the absorptionbodies provided on or in the absorption layer 9 are satisfactorilywetted by the printing substance 2. Here, also, the absorption bodies 4are chosen to be as small as possible, i.e. so far as possible very muchsmaller than 1 μm in diameter, and can for example consist of carbonblack particles.

In order to achieve as good as possible and largely punctiformabsorption, the proportion of absorption bodies 4 in the absorptionlayer 9 is as high as possible. The absorption layer 9 advantageouslyeven consists completely of pressed absorption bodies. It is alsopossible to embed the absorption bodies in an organic or inorganicpolymer matrix or in an elastomer matrix, such as e.g. rubber.

Because the laser light is directly absorbed by the absorption body, andonly a very small quantity of heat is transferred to adjacent absorptionbodies or the absorption layer, in general energy of less than 5 μJ isalready sufficient for a printing point with a diameter of >200 μm. Fora printing point with a diameter <40 μm, 1 μJ of energy is evensufficient. In comparison with this, an electrothermal heating element,such as is used for example in the bubble-jet process mentioned at thestart, requires between 20 and 30 μJ of energy for a printing point witha diameter of approximately 100 μm. The energy consumption in the caseof direct laser absorption, i.e. the energy required per printing pointin the case of a printing process without absorption bodies, is ingeneral even higher.

The printing substance 2 can either be present in cells or in otherregular structures, such as are known for example from DE 197 46 174,mentioned at the start, or else be designed as a homogeneous colourfilm. The advantage of the continuous homogeneous colour film is thatthe laser beam does not have to be directed into preset structures, suchas occurs for example through the cells, but can be directedindependently to any desired position, with the result that the printingprocess operates independently of resolution. However, when cells or thelike are not used, the printing substance is no longer guided, with theresult that the printing points lose some sharpness. In order to stillbe able to detach defined drops from an essentially smooth colouredsurface by direct laser firing, according to the invention, besides theuse of absorption bodies, which as described make possible the defineddetachment of a printing point with low energy, further conditions arealso observed. Thus the ink viscosity, the ink-layer thickness, thelaser exposure time, the laser energy and the laser energy density inthe focus as well as the correct blowing agent ratio have to be set inthe printing substance. If the exposure time can be kept very short,i.e. very much less than 1 μs, and the process energy in the laser focuslies clearly above the threshold value (very much greater than 500kW/cm²) at a laser wavelength of approximately 1 μm, the ejection of adrop occurs due to the very high rate of heating up in such a short timethat a shock wave spreads perpendicular to the surface in the directionof the imprinting material and the adjacent printing substance is a kindof guide because of its mass moment of inertia.

This has been once again illustrated in FIGS. 5A and 5B. FIG. 5A showsthe situation in which the preset printing parameters are observed. Thelaser beam 3 is focussed into the printing substance 2, with the resultthat a drop 5 detaches from the printing substance 2 and generates adefined sharp printing point 7 on the imprinting material 6. In contrastto this, FIG. 5B shows the situation in which the preset parameters arenot observed. As the energy used here is clearly higher and the wholearea surrounding the focus is also heated up, the result here is anuncontrolled ejection of a drop 5, with the result that a much largerprinting point 7 forms on the imprinting material 6.

FIGS. 6A and 6B show how, according to the invention, the printing-pointsize can be changed by variation of individual printing parameters.Tests have shown that the size of the printing point and the colourdensity of the printing point increase with increasing exposure time,i.e. with the pulse duration of the laser. The size of the printingpoint can thus be varied in almost infinitely small steps. Differencesin brightness in the image to be printed can therefore be represented byprinting points of different size. Overall it is possible, through theprinting process according to the invention, to clearly improve theprint format. The resolution can be increased almost at will, as theprinting process is not connected with any kind of preset raster.

FIG. 7 shows a further preferred version of the invention. It is clearthat the ink carrier 1 has a surface with recesses and/or elevations.The recesses are here applied in the form of grooves 10. The recessesand/or elevations 10 preferably have a diameter of less than 25 μm. Therespective depth and height of the recesses and/or elevations is alsopreferably less than 25 μm. The structure is thus extremely fine. Theprinting substance forms a continuous surface on the structure of theink carrier, as also shown in FIG. 6. The laser beam 3 passes frombehind through the transparent ink carrier 1. The microstructure appliedto the ink carrier 1, which can also have the form of cells and/orgrooves, ensures that the shock wave conveyed through the absorptionbodies runs as far as possible perpendicular to the surface of the inkcarrier 1. Surprisingly in several tests it was established that, bysuitable “roughening” of the surface of the ink carrier, the quality ofthe generated printing point 7 on the imprinting material 6 can beconsiderably enhanced.

At the same time almost any desired fine resolution can be achieved.This is an essential feature distinguishing this version from thoseprinting processes in the state of the art which use cells on the inkcarrier. The known cells are distinctly larger, with the result that theprinting substance is situated exclusively in the cells (any supernatantprinting substance is scraped off). In contrast to this themicrostructure here is chosen so that the printing substance forms acontinuous layer beyond the structure. The structure is thus completelycovered by the printing substance.

The microstructure is irregularly structured in a particularly preferredversion, i.e. the individual recesses and/or elevations vary in theirextent and depth. A variation of approximately 10–50% of the dimensionof the recess and/or elevation is advantageously chosen. The elevationsand/or recesses are expediently formed such that the shock wave producedupon striking of the electromagnetic wave is advantageously shaped, withthe result that the cleanest possible printing point forms.

FIG. 8 shows a further version of the present invention. Herelight-focussing elements 11 are applied to the ink carrier 1. Theselight-focussing elements form a microlens system which ensures that theincident focussed light beam is once again split into several smallfocal spots, so-called “hot spots”. The result of this is that theenergy density is further increased at certain points, with the resultthat the energy to be introduced overall can be still further reduced.The individual light-focussing elements 11 are smaller than the focus ofthe laser, i.e. smaller than 30 μm, preferably smaller than 15 μm, andparticularly preferably smaller than 10 μm. The light-focussing elementscan, in the arrangement shown in FIG. 7 with a transparent ink carrier1, in principle be arranged both on the side of the ink carrier 1 facingaway from the printing substance 2, and also on the side of the inkcarrier 1 wetted with printing substance 2. Of course thelight-focussing elements can also be integrated into the ink carrier 1.

FIGS. 9A and 9B show versions with different arrangements of thelight-focussing elements 11.

Finally, FIG. 10 shows a “negative” arrangement of the printing processaccording to the invention. The laser beam 3, which is focussed on theink carrier 1, can be seen. The cylindrical ink carrier 1 is in contacton one side with a dipping roller 14 which in turn dips into a bath withprinting substance 2. Upon rotation of the dipping roller and the inkcarrier 1 it is ensured that the surface of the ink carrier 1 is coveredwith printing substance 2.

At certain points individual areas of the printing-substance layer areremoved from the ink carrier by means of the laser beam 3. As the inkcarrier 1 additionally comes into contact with the imprinting materialrunning over a printing roller 13, that part of the printing substancewhich has not been removed by the laser is transferred onto theimprinting material 6. Unlike the process described at the start, herethe laser is focussed on the areas which are not to contribute to thetransfer of the printing substance.

It is understood that the additional features of the versions describedcan also be advantageously realized in other printing processes or inother printing machines.

Finally, FIG. 11 shows a further version of the present invention inwhich, between an inking roller with an absorption layer, onto which thelaser is directed, and the printing material guided over a supportingroller, or the web to be imprinted, a continuous blanket guided overseveral rollers is provided as print transfer medium. This can inparticular be a rubber blanket, rubber blankets having the favourableproperty that they can be guided over particularly narrow radii ofcurvature.

The supporting or carrying roller for the rubber blanket, arrangedopposite the inking roller, can then have a relatively small diameter,such that it is possible to allow the laser to strike the absorptionlayer of the inking roller at a relatively steep angle and yet in veryclose proximity to the rubber blanket, with the result that the drops ofink thereby detached have only a very short distance to travel beforethey reach the rubber blanket. The rubber blanket is then guided over aprinting roller, which lies opposite a supporting roller for theprinting web, with the result that the ink can be transferred from therubber blanket onto the printing web.

Any ink residues still adhering to the rubber blanket after the printingprocedure are then removed by a cleaning roller.

1. A printing process for the transfer of liquid printing substance (2)from an ink carrier (1) to an imprinting material (6), in which, withthe help of an energy-emitting apparatus, which, during a processperiod, emits energy in the form of electromagnetic waves (3), such thatwith the help of absorption bodies (4), energy is transferred from theelectromagnetic waves (3) into the printing substance (2) causing achange in volume and/or position, so that printing substance is firsttransferred to a print transfer medium and then from the print transfermedium to the imprinting material, said electromagnetic waves beingdirected so that they avoid passage through the transfer medium.
 2. Aprinting process according to claim 1, characterized in that whereinabsorption bodies (4) are used which that are smaller than thewavelength of the electromagnetic waves (3).
 3. A printing processaccording to claim 1, wherein absorption bodies (4) are used that aresmaller than 1/10 of the wavelength of the electromagnetic waves (3). 4.A printing process according to claim 1, wherein absorption bodies (4)are used that are smaller than 1/50 of the wavelength of theelectromagnetic waves (3).
 5. A printing process according to claim 1wherein the apparatus emits energy in the form of laser light (3).
 6. Aprinting process according to claim 5 wherein printing-point size iscontrolled by a profile of the laser light.
 7. A printing processaccording to claim 1 wherein a printing substance (2) is used whichcontains absorption bodies (4).
 8. A printing process according to claim7, characterized in that the absorption bodies (4) are also used as dye.9. A printing process according to claim 1 wherein the absorption bodies(4) absorb essentially all the light wavelengths.
 10. A printing processaccording to claim 1 wherein the absorption bodies (4) absorbessentially only the radiation with a wavelength or in a wavelengthrange which corresponds to the wavelength or wavelength range of theelectromagnetic waves 3 emitted by the energy-emitting apparatus.
 11. Aprinting process according to claim 10 wherein the absorption bodies (4)are accelerated in the direction of the imprinting material (6) by theelectromagnetic waves (3) of the energy-emitting apparatus.
 12. Aprinting process according to claim 1 wherein an ink carrier (1) is usedhaving a surface provided to receive the printing substance (2), whichsurface has absorption bodies (4) which form a solid layer.
 13. Aprinting process according to claim 1 wherein printing-point size iscontrolled by the quantity of energy released by the energy-emittingapparatus.
 14. A printing process according to claim 13 wherein thequantity of energy released by the energy-emitting apparatus iscontrolled by a process period.
 15. A printing process according toclaim 1 wherein differences in brightness of the image to be printed areachieved by variation of the printing-point size.
 16. A printing processaccording to claim 1 wherein the process period is shorter than 1 μs.17. A printing process according to claim 1 wherein the process periodis shorter than 250 ns.
 18. A printing process according to claim 1wherein the process period is shorter than 100 ns.
 19. A printingprocess according to claim 1 wherein the process period is shorter than50 μs.
 20. A printing process according to claim 1 wherein during theprocess period an energy density higher than 500 kW/cm² is generated atthe absorption body (4).
 21. A printing process according to claim 1wherein during the process period an energy density higher than 2 MW/cm²is generated at the absorption body (4).
 22. A printing processaccording to claim 1 wherein during the process period an energy densityhigher than 10 MW/cm² is generated at the absorption body (4).
 23. Aprinting process according to claim 1 wherein the absorption bodies (4)are heated during the process period with an average heating rategreater than 10⁹ K/s.
 24. A printing process according to claim 1wherein the absorption bodies (4) are heated during the process periodwith an average heating rate greater than 10¹¹ K/s.
 25. A printingprocess according to claim 1 wherein the thickness of the printingsubstance (2) on the ink carrier is less than 50 μm.
 26. A printingprocess according to claim 1 wherein the printing substance (2) isselected so that the viscosity lies between 0.05 and 0.5 Pas.
 27. Aprinting process according to claim 1 wherein for the production of aprinting point with a diameter greater than 100 μm, an energy of notmore than 10 μJ, is transferred.
 28. A printing process according toclaim 1 wherein through the change in volume and/or position of theprinting substance some of the printing substance is removed from theink carrier and is at least partly transferred to the imprintingmaterial.
 29. The printing process according to claim 1 where the printtransfer medium can be roller guided about a radius.
 30. The printingprocess according to claim 1 where the print transfer medium is acontinuous blanket.
 31. The printing process according to claim 1wherein printing is line-by-line, areas to be printed within a linebeing formed by line segments of any choosable length and any choosableposition.
 32. The printing process according to claim 31 wherein theline-by-line printing takes place with a continuous wave laser, whichtravels line-by-line down the lines of print following a preset rasterand which can be switched on and off or revealed or masked as desiredalong a line.
 33. A printing machine for the transfer of liquid printingsubstance (2) to an imprinting material, said machine having an inkcarrier (1) for holding ink and an energy-emitting apparatus, foremitting energy in the form of electromagnetic waves during a processperiod, which energy-emitting apparatus is arranged such that energy canbe transferred in a targeted manner onto certain areas of the inkcarrier (1), wherein absorption bodies (4) on the ink carrier areprovided for absorbing the energy such that with the help of theabsorption bodies (4), energy is transferred from the electromagneticwaves (3) into the printing substance (2) causing a change in volumeand/or position of printing substance causing printing substancetransfer, said machine being arranged so that printing substance isfirst transferred to a print transfer medium and then from the printtransfer medium to the imprinting material, said energy-emittingapparatus being arranged such that said electromagnetic waves aredirected so that they avoid passage through the transfer medium.
 34. Aprinting machine according to claim 33, wherein the energy-emittingapparatus is a laser.
 35. A printing machine according to 33, whereinthe absorption bodies (4) are of a size which is smaller than 1 μm. 36.A printing machine according to 33, wherein the absorption bodies (4)are of a size which is smaller than 200 nm.
 37. A printing machineaccording to claim 33, wherein the absorption bodies (4) are of a sizewhich is between 10 and 50 nm.
 38. A printing machine according to claim33, wherein the absorption bodies consist of carbon black particles,titanium nitride or mixtures thereof.
 39. A printing machine accordingto claim 33, wherein the absorption bodies (4) are arranged in anabsorption layer (9) arranged on the ink carrier (1).
 40. A printingmachine according to claim 39, wherein the absorption layer (9) consistsof pressed absorption bodies (4).
 41. A printing machine according toclaim 39, wherein the absorption bodies (4) are embedded in an organicor inorganic polymer matrix.
 42. A printing machine according to claim39, wherein at the ink carrier (1) has a surface structure whichconsists of recesses and/or elevations.
 43. A printing machine accordingto claim 33, wherein the proportion of the absorption bodies (4) in theabsorption layer (5) is greater than 40 wt percent.
 44. A printingmachine according to claim 33, wherein light-focussing elements areapplied to the ink carrier.
 45. A printing machine according to claim44, wherein the light focusing elements are formed by a flexible polymerfilm.
 46. A printing machine according to claim 33, wherein the inkcarrier is transparent and the light-focussing elements are integratedinto the ink carrier.
 47. The printing machine according to claim 33where the print transfer medium can be roller guided about a radius. 48.The printing machine according to claim 33 where the print transfermedium comprises rubber.
 49. The printing machine according to claim 33where the print transfer medium is a cylinder comprising rubber.